A 50-year-old man sustained posterior dislocation of left hip in an accident. Dislocation was reduced after 3 days. He started complaining of pain in left hip after 6 months. X-ray of the pelvis were normal. The most relevant investigation at this stage will be-

It is a case of avascular necrosis of the femoral head which is one of the late complications of posterior dislocation of the hip. If the dislocation is reduced early, avascular necrosis develops in at least 10% of the cases of dislocation. If reduction is delayed for more then few hours (as with this patient) the figure rise to 40%. Pain is a common presentation of avascular necrosis of head. X-ray of the patient is normal because in the early stages bones are usually normal. The first X-ray signs probably appear only 6-9 months after the occurrence of bone death. MRI shows characteristic changes in the marrow long before the appearance of X-ray signs. Therefore MRI is the most reliable way of diagnosing marrow changes and bone ischaemia (Avascular necrosis) at a comparatively early stage.